There are currently two basic types of connectors found in common use with wiring devices, such as switches and receptacles, which are used in residential, commercial, and industrial applications.
The first type connector commonly found on such wiring devices is characterized by a screw which secures an electrical conductor to the wiring device. In such devices, the screw is threaded into a threaded hole within the electrical contact member of the wiring device. An electrical conductor is wrapped around the shank, or body, of the screw. The screw is then driven into the device. As the screw is driven into the device, the head of the screw presses the electrical conductor against the contact member of the device. With the conductor so secured to the device, it is then ready for service.
This method of terminating an electrical conductor on a wiring device provides certain advantages. One advantage is that a certain and firm contact is initially made between the electrical conductor being terminated and the electrical contact member of the wiring device. Another advantage of this configuration is the simplicity and low cost of manufacturing such a device.
However, the advantage of a firm initial contact cited above is also a distinct disadvantage. As electrical conductors are put under load, they heat up. As they heat up, they expand. Since the position of the screw, and therefore the screw head holding the conductor, is fixed, it does not expand with the conductor. The conductor expands against the connector screw head and plastic deformation of the relatively soft conductor results. After the electrical current load is removed, the conductor cools off. As the conductor cools off, it contracts creating a deformation space or gap between the surface of the conductor and the screw head. This space or gap results in decreased contact between the conductor and the connector. Less contact between the conductor and the connector results in more resistance at the connection. More resistance at the connection results in higher temperatures when the circuit is loaded again. Each cycle results in higher temperatures which in turn cause the conductor to flow and further deform with more dramatic results. Each time the cycle repeats, the resistance is increased, thereby resulting in an accelerated deterioration of the contact between the connector and the conductor, with catastrophic failure being the ultimate result.
A second type of connector has been developed to eliminate some of the problems created when the electrical conductor heats and expands. This type connection is commonly known as a "push-in" type system. This system utilizes a tooth-edge contact that is pressed against the conductor by means of a spring member rather than by a fixed screw head as described above. In this device, the connector is placed between the fixed surface of the contact member on the wiring device and a spring loaded, floating, tooth-edge securing member. Again, as the conductor is placed under load, it heats and expands. However, with the conductor secured to the wiring device by the floating, spring loaded member, there is no plastic deformation because the connector "gives" and moves with the conductor as the expanding conductor presses against the spring loaded connector member. Because the connector "gives" to accommodate conductor expansion, plastic deformation does not occur. When the load is removed, the conductor cools and contracts and the spring loaded member again reacts to changing conductor dimensions and remains in contact with the contracting conductor thereby eliminating the deformation induced gap that forms when screw type connectors are attached to a conductor. Because there is no deformation space or gap, there is no increase in the resistance of the connection and there is no accelerated deterioration of the connection.
One inherent disadvantage of the tooth-edge contact connector is that the conductor and the connector are in contact only at the points of the teeth, thereby resulting in only minimal contact between the conductor and the connector. Such a contact mechanism can result in conductor dislocation with a resultant connection deterioration or the complete severance of the junction formed by the wiring device and the electrical conductor.